U.S. patent number 5,169,736 [Application Number 07/727,563] was granted by the patent office on 1992-12-08 for electrochemical secondary element.
This patent grant is currently assigned to Varta Batterie Aktiengesellschaft. Invention is credited to Rainer Bittihn, Rudolf Herr, Detley Hoge.
United States Patent |
5,169,736 |
Bittihn , et al. |
December 8, 1992 |
Electrochemical secondary element
Abstract
An electrochemical secondary element with a non-aqueous
electrolyte, whose charge/discharge mechanism is predicated upon
the reversible intercalation of active Li+ ions in electrode
materials having an open skeletal structure. The positive electrode
is essentially a lithium-manganese spinel of the type Li.sub.q
M.sub.x Mn.sub.y O.sub.z which, in addition to satisfying the
expressions 0<x<0.6 and 1.4<y<2.0, can also contain up
to 30% of other metals such as Co, in addition to Mn. The negative
electrode is based on a carbon product obtained by pyrolysis from
organic compounds, and serves as the recipient substance for the
Li+ ions. By limiting the Li concentration in the spinel lattice to
a range in which the numerical values of the variable q lie between
0 and 1.3, a lithium-poor manganese spinel is obtained. During
discharging of the preponderant portion of the secondary element's
capacity, the result is that a cell potential greater than 3 volt
is obtained, and high cycling stability is also manifested.
Inventors: |
Bittihn; Rainer (Idstein,
DE), Hoge; Detley (Kelkheim, DE), Herr;
Rudolf (Kelkheim, DE) |
Assignee: |
Varta Batterie
Aktiengesellschaft (Hanover, DE)
|
Family
ID: |
6411902 |
Appl.
No.: |
07/727,563 |
Filed: |
July 9, 1991 |
Foreign Application Priority Data
Current U.S.
Class: |
429/300; 429/223;
429/224; 429/302; 429/217 |
Current CPC
Class: |
H01M
4/131 (20130101); H01M 10/0563 (20130101); H01M
4/133 (20130101); H01M 10/05 (20130101); H01M
4/485 (20130101); H01M 4/587 (20130101); H01M
10/399 (20130101); Y02P 70/50 (20151101); H01M
2004/027 (20130101); H01M 2300/0048 (20130101); H01M
2300/0025 (20130101); H01M 2300/0082 (20130101); Y02E
60/10 (20130101); H01M 2004/028 (20130101) |
Current International
Class: |
H01M
10/40 (20060101); H01M 4/48 (20060101); H01M
4/02 (20060101); H01M 10/36 (20060101); H01M
10/39 (20060101); H01M 4/58 (20060101); H01M
006/14 (); H01M 004/58 () |
Field of
Search: |
;429/194,217,197,198,223,192 |
References Cited
[Referenced By]
U.S. Patent Documents
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|
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4132619 |
January 1979 |
Klein et al. |
4507371 |
March 1985 |
Thackeray et al. |
4956247 |
September 1990 |
Mizazaki et al. |
4975346 |
December 1990 |
Lecerf et al. |
|
Foreign Patent Documents
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|
|
|
|
|
|
63-218156 |
|
Sep 1988 |
|
JP |
|
63-221559 |
|
Sep 1988 |
|
JP |
|
1-109662 |
|
Apr 1989 |
|
JP |
|
Primary Examiner: Chaudhuri; Olik
Assistant Examiner: Nuzzolillo; M.
Attorney, Agent or Firm: Weiser & Stapler
Claims
What is claimed is:
1. An electrochemical secondary element having a positive
electrode, a negative electrode, and a non-aqueous electrolyte,
which element comprises an electrode material which forms an open
grid or skeleton host structure acting as a recipient substance for
alternatively taking up and releasing electrochemically active
cations during charging and discharging, the recipient substance of
the positive electrode being an oxidic material of spinel structure
whose composition conforms to the general formula M.sub.x Mn.sub.y
O.sub.z, in which
M=a cation of at least one metal from Groups IIa, IVa, Va, IVb, Vb,
VIb or VIIIb,
x=a number greater than 0 and less than 0.6, and
y=a number greater than 1.4 and less than 0.2, wherein the ratio y
to z lies between 0.3 and 0.6, and
wherein the taking up of a cation A of a Group Ia element within
the host structure produces a spinel-type electrode of the general
composition A.sub.q M.sub.x Mn.sub.y O.sub.z in which A varies as a
function of a variable q, wherein q lies between 0 and 1.3, and
wherein the recipient substance of the negative electrode is a
carbon product formed by pyrolysis of organic compounds.
2. The electrochemical secondary element of claim 1 wherein A is
constituted of Li+.
3. The electrochemical secondary element of claim 1 wherein the
recipient substance of the carbon product forming the negative
electrode is a needle coke.
4. The electrochemical secondary element of claim 1 wherein the
electrolyte includes a Li salt with an organic solvent.
5. The electrochemical secondary element of claim 4 wherein the
electrolyte is immobilized by a polymeric matrix solvent, or by the
addition of an inorganic oxygen-containing compound which gels in
the solvent.
6. The electrochemical secondary element of claim 5 wherein the
polymeric base is polyethylene oxide.
7. The electrochemical secondary element of claim 5 wherein the gel
is gelated by means selected from the group consisting of
SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2, MgO, B.sub.2 O.sub.3,
Na.sub.2 SO.sub.4 and AlPO.sub.4.
8. The electrochemical secondary element of claim 1 wherein the
electrolyte is constituted of a high temperature melt containing a
lithium salt.
9. The electrochemical secondary element of claim 1 wherein the
electrolyte is a room-temperature salt melt containing a Li
salt.
10. An electrochemical secondary element having a positive
electrode, a negative lithium-intercalating carbon electrode, and a
non-aqueous electrolyte, which element comprises an electrode
material which forms an open grid or skeleton hose structure acting
as a recipient substance for alternatingly taking up and releasing
electrochemically active cations during charging and discharging,
the recipient substance of the positive electrode being an oxidic
material of spinel structure whose composition conforms to the
general formula M.sub.x Mn.sub.y O.sub.z, in which
M=a cation of at least one metal from Groups IIa, IVa, Va, IVb, Vb,
VIb and VIIIb,
x=a number greater than 0 and less than 0.6, and
y=a number greater than 1.4 and less than 2.0, wherein the ratio y
to z lies between 0.3 and 0.6, and
wherein the taking up of a cation A of a Group Ia element within
the host structure produces a spinel-type electrode of the general
composition A.sub.q M.sub.x Mn.sub.y O.sub.z in which A varies as a
function of a variable q, wherein q lies between 0 and 1.3, and
wherein the recipient substance of the negative electrode is a
carbon product formed by pyrolysis of organic compounds.
Description
BACKGROUND OF THE INVENTION
This invention relates to a secondary element having a positive
electrode, a negative electrode, and a non-aqueous electrolyte, in
which the material of the respective electrodes forms an open grid
or skeleton structure. This makes the electrode structure capable
of acting as a "host" for a recipient compound, for alternately
accepting and releasing electrochemically active cations during
charging and discharging.
An important impetus for this invention was the recent, great
increase in demand for batteries having high energy density and low
weight, such as had already been achieved particularly with the
lithium systems, but which are also rechargeable. This requires,
among other things, that the electrodes be chemically stable in
contact with the electrolyte.
Lithium electrodes do not meet these requirements when in use over
extended periods of time, even in organic electrolytes with an
aprotic solvent, because their cycling stability is well known to
be sharply limited. This drawback can be overcome by alloying
lithium with an alkaline earth metal or earth metal, preferably
aluminum. By so doing, the reduced energy content of the alloyed
electrode is offset by the benefit of better rechargeability and
higher mechanical strength.
Another recent approach to improve the reversibility of lithium
electrodes involves intercalation compounds. With such compounds,
the cell's electrode incorporates a material of predetermined
structure which form an appropriate "host" or recipient grid for
electrochemically active species of ions that are present in the
electrolyte. These ions, in this instance Li+, are either stored or
released depending upon the polarity of an externally applied
potential. During discharge, the electromotive force which is
produced, and which manifests itself in the tendency to again
reverse the forced intercalation or deintercalation, is used for
current production.
From the outset, carbon of predetermined structural characteristics
has proven suitable as the electrode material dopable with Li+ ions
for both electrode polarities. For example, the electrodes of the
electrochemical battery disclosed in German patent publication
(DE-OS) 3,231,243 involve such products formed from active carbon.
According to European patent application (EP-A) 165,047, the carbon
material can be a pseudographite of predetermined crystalline size
and with a lattice expanded in the direction of the c-axis which is
obtained by pyrolysis from aromatic hydrocarbons. In a secondary
battery disclosed in European patent application (EP-A) 201,038,
which has as an electrolyte a solution of a lithium salt in a
non-aqueous solvent, such a pseudographite forms the negative
electrode, and a metal chalcogenide which is also capable of being
doped forms the positive electrode.
The intercalation capability of metal chalcogenides, e.g. of
WO.sub.3 or TiS.sub.2, as well as of certain synthetic mixed oxides
disclosed in U.S. Pat. No. 4,668,595, is based upon their well
defined lattice layers. The same printed publication also discloses
chemically stable n-type material in the form of a carbon product,
which is obtained from high molecular weight components of crude
oil by a controlled coking process. In such case, for the purpose
of incorporating metal cations, particularly Li+, a certain
irregularity or lack of organization in the fine structure of the
product is desired.
Finally, U.S. Pat. No. 4,507,371 discloses that, in rechargeable
cells, host oxides or sulfides having crystalline chemistry of the
spinel type can be used as either cathode or anode materials, and
even as electrolyte if no electron conductivity is present. These
spinel structures have high inherent stability, or can be
stabilized if needed by the incorporation of certain cations such
as Mg.sup.2+, Zn.sup.2+, Cd.sup.2+.
In particular, German patent publication (DE-OS) 3,736,366
discloses that pure lithium-manganese spinel, in which lattice the
Li ions have high mobility, can be produced through the
transformation of manganese dioxide (MnO.sub.2) with lithium salts
at only moderately high temperatures of 300.degree. C. to
400.degree. C. This is what makes such spinels suitable as the
active cathode material, particularly for rechargeable galvanic
elements. In the charged state, such spinels have the formula
LiMn.sub.2 O.sub.4, and in the discharged state, the formula
LiMnO.sub.2. Through acid treatment, the lithium manganese spinel
can be transformed into a lithium-poor compound without
modification of its spinel structure, and with only a minor
contraction of the cubic lattice.
In all previously known secondary elements with cathodes including
a material with spinel structure, the associated anode is either a
lithium alloy, an electrically conductive polymer doped with
lithium ions such as polyacetylene or polyparaphenylene, or an
intermediate layer compound of the TiS.sub.2 -type, which has
lithium ions in the intermediate layer spaces, or else it is formed
of a spinel type, like the cathode.
SUMMARY OF THE INVENTION
The present invention has as an object to provide an
electrochemical secondary element of high energy density, whose
charge/discharge mechanism is based upon alternating intercalation
and de-intercalation, preferably of Li+ ions in the materials of
the positive and negative electrodes. It is another object to
provide electrode structures which provide good chemical resistance
to the electrolyte, and high cycling stability.
This object and others which will appear are achieved in accordance
with the present invention by means of a secondary element having a
positive electrode, a negative electrode, and a non-aqueous
electrolyte, which element comprises an electrode material which
forms an open grid or skeleton structure enabling it to act as a
recipient substance for alternatingly taking up and releasing
electrochemically active cations during charging and
discharging.
The recipient substance of the positive electrode is preferably an
oxidic material of spinel structure whose composition conforms to
the general formula M.sub.x Mn.sub.y O.sub.z, in which
M=a cation of at least one metal from Groups IIa, IVa, Va, IVb, Vb,
VIb or VIII,
x=a number between 0 and 0.6, and
y=a number between 1.4 and 2.0, wherein the ratio y to z lies
between 0.3 and 0.6.
The taking up of a cation A of a Group Ia element within the host
structure produces a spinel-type electrode of the general
composition A.sub.q M.sub.x Mn.sub.y O.sub.z in which A varies as
variable q, and the recipient substance of the negative electrode
is a carbon product formed by pyrolysis of organic compounds. For
further details, reference is made to the description which is
provided below, in view of the following drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows variation in potential during a charge/discharge cycle
for a positive spinel electrode measured relative to a Li/Li+
counter-electrode.
FIG. 2 shows variation in the capacity of two positive spinel
electrodes embodying the present invention during cyclical
charging/discharging over 15 cycles, again relative to a Li/Li+
electrode.
FIG. 3 shows characteristic variation during a charge/discharge
cycle for an assembled secondary cell embodying the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, it has been found that
the above requirements can be met in large measure by a secondary
cell having a positive electrode which includes as a recipient
substance an oxygen-containing material which exhibits a fine
structure, like that of a spinel, and whose composition corresponds
to the general formula M.sub.x Mn.sub.y O.sub.z in which:
M=a cation of at least one metal from the Groups IIa, IVa, Va, IVb,
Vb, VIb, or VIIIb;
x=a number between 0 and 0.6; and
y=a number between 1.4 and 2.0, wherein the ratio y to z iu between
0.3 and 0.6.
When the recipient structure takes up a cation A of an element from
Group Ia, an electrode is obtained of the general composition
A.sub.q M.sub.x Mn.sub.y O.sub.z and of the spinel type, in which A
varies as a function of the variable q. The recipient substance of
the negative electrode is n carbon product obtained through
controlled thermal decomposition of organic compounds. The
electrochemically active cation A, which is capable of being
intercalated in both electrode structures, is preferably Li+.
In accordance with the present invention, the positive electrode of
the secondary element includes a lithium-manganese spinel, Li.sub.q
M.sub.x Mn.sub.y O.sub.z, in which, depending upon the value of the
parameters 0<x<0.6 and 1.4<y<2.0, up to about 30% of
the manganese can be replaced by other metals. Replacement metals
are primarily the main elements Mg, Sn, Pb, Sb and Bi from Groups
IIa, IVa and Va of the periodic system, as well as the transition
elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Co and Ni from
Groups IVb, Vb, VIb and VIIIb.
The partial replacement of Mn-ions by M-cations in the spinel
lattice sometimes requires certain defects in the lattice
structure, with the result being that the recipient oxide is not
always, and indeed only rarely is, a stoichiometric compound, and
that the M-cations are introduced into the lattice with different
valences than the Mn-ions. Correspondingly, the parameters x, y, z
have a certain range of variation but are fixed for each individual
compound. If the electrochemically active ion species A is diffused
into the spatial or skeletal structure of the host oxide under the
influence of an electric field, then the concentration of A, or
Li+, can vary over a wide range.
In accordance with the present invention, the preferred numerical
value of q lies between 0 and 1.3. Due to this limitation, cathodes
embodying the invention are characterized as lithium-poor manganese
spinels in comparison, for example, to the Li.sub.1+x Mn.sub.2
O.sub.4 -phase (where x corresponds to q), which is disclosed in
U.S. Pat. No. 4,507,371. However, these are distinguished by the
fact that the overwhelming portion of their capacity can be derived
at potentials greater than 3 volts as compared with Li/Li+.
Furthermore, the intercalation of the Li+ ions takes place with a
high degree of reversibility. As will be shown, a particularly
suitable material for the positive electrode of a secondary element
embodying the present invention has proven to be a
cobalt-containing lithium-manganese spinel of the composition
LiCo.sub.0.24 Mn.sub.1.76 O.sub.4.065.
As the framework of the negative electrode, which functions as a
recipient substance for the electrochemically active cation species
A, there is used according to the present invention a carbon
product which is produced from selected organic compounds through a
coking or pyrolysis process. A desirable process for this purpose
is a slowed coking; the so-called "delayed coking process". By this
process, residues from petroleum refining, which are used as the
raw material, are placed in an oven and heated to about 500.degree.
C. However, the transition speed, or dwell time in the oven, is so
chosen that the coke deposition takes place only in the coking
chamber which follows, and which is operated alternately with a
second chamber. Removal of the coke from these chambers takes place
with the assistance of a hydraulic scraping device after
by-products including volatile hydrocarbons have been driven off
through the introduction of steam.
A carbon product which is particularly suitable for the purposes of
the present invention is so-called needle coke. This is a special
coke which has previously been used in the steel industry for the
production of high quality graphite electrodes for electric
furnaces. Needle coke is also a product of the above-described
delayed coking process, but contains contaminants such as thermal
tars, decant oils, or bituminous coal tar pitch, which are all
based on highly aromatic hydrocarbon compounds. For further
information regarding this process, as well as for typical needle
coke specifications, reference is made to an article by H. M.
Feintuch, J. A. Bonilla and R. L. Godino, in the Handbook Of
Petroleum Refining Processes, R. A. Meyers, McGraw Hill, New York,
pages 7.1 to 7.61 (1986).
The non-aqueous electrolyte of a secondary element according to the
present invention can be liquid, paste-like or solid. Preferably,
the electrolyte includes a Li salt with an organic solvent, and is
in liquid form. Useful for this purpose are known electrolyte salts
with one of the anions ClO.sub.4 --, BF.sub.4 --, AsF.sub.6,
CF.sub.3 SO.sub.3 --, PF.sub.6 --, J--, or AlClO.sub.4 --. As a
water-free solvent for these salts, there can be used both
individually and in mixture with others, an organic solvent of the
group tetrahydrofuran, 2-methyltetrahydrofuran, acetonitrile,
propionitrile, 4-methyl-2-pentanone, butyronitrile, valeronitrile,
benzonitrile, 1,2-dichlorethane, .gamma.-butyrolactone,
dimethoxyethane, methylformiate, propylene carbonate, ethylene
carbonate, vinylene carbonate, dimethylformamide, sulfolane,
3-methylsulfolane, trimethylphosphate, and other like organic
solvents.
In a preferred secondary cell embodying the present invention, the
electrolyte is immobilized by inorganic oxygen-containing compounds
such as SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2, MgO, B.sub.2
O.sub.3, Na.sub.2 SO.sub.4 or AlPO.sub.4, which gel with the
solvent in surface rich forms. Particularly suitable are aprotic
solvents such as propylene carbonate, acetonitrile,
.gamma.-butyrolactone, nitromethane, tetrahydrofuran,
methyltetrahydrofuran, dimethoxyethane or dioxolane. The finished
electrodes are of a pasty or semi-solid consistency.
A different way of immobilizing the electrolyte involves forming
polyether-complexes of the alkaline salts (e.g., with polyethylene
oxide) which have the properties of a solid ion conductor. In such
case, the lithium salt used as the electrolyte component is an
ingredient of a polymeric electrolyte matrix with polyethylene
oxide as the framework. Ceramic alkaline ion conductors can also be
used as solid electrolytes.
Finally, the electrolyte of the secondary element embodying the
present invention can also include molten salts. These include, for
example, LiAlCl.sub.4 which melts at 150.degree., or an eutectic
mixture of LiCl and KCl, with a melting point of 352.degree. C.
However, particularly preferred electrolytes are so-called "room
temperature salt melts", which here function as a solvent for a
conventional Li-conductive salt. As examples, there are listed the
following salt mixtures: 1 methyl-3-ethylimidazolium
chloride/AlCl.sub.3, N-butylpyridinium chloride/AlCl.sub.3,
phenyltrimethylammonium chloride/AlCl.sub.3.
Neither the electrode materials nor the electrolytes of the
secondary elements which embody the present invention pose
particular difficulties for the construction of practical cells, or
to the battery assembly process. For example, the electrodes can be
produced in compact form, which makes them easily usable for
assembly. If desired, the electrodes can be united simply by means
of an adhesive. For tight constructions, and in conjunction with
liquid electrolytes, the electrodes can be electrically isolated
from each other with a separator material of the type which is
conventionally used in lithium cells, such as polypropylene for
example.
Particularly desirable is a strengthening of the electrodes by
means of electrically conductive metal support structures,
especially when these simultaneously perform the current take-off
function. In a particularly desirable embodiment of the secondary
element of the present invention, the negative electrode has a
take-off conductor of nickel or high-grade steel, and the positive
electrode has a take-off conductor of aluminum or high-grade
steel.
Referring now to the drawings the voltage curve of FIG. 1 was
obtained with a pure lithium-manganese spinel of the composition
LiMn.sub.2 O.sub.4, which was derived from the general formula
Li.sub.q M.sub.x Mn.sub.y O.sub.z for the case q=1, x=0 and y=2.
The low lithium content (0<q<1.3) according to the present
invention is responsible for an exceptionally high voltage level of
about 4 volts during discharge.
As shown in FIG. 2, spinel cathodes according to the present
invention distinguish themselves in cyclical operation (n=number of
cycles) through only slight capacity decline (capacity C in mAh).
As for cycling stability, even the LiMn.sub.2 O.sub.4 spinel was
surpassed by the previously mentioned LiCo.sub.0.24 Mn.sub.1.76
O.sub.4.065 modification. This desirable behavior results directly
from the almost unimpeded take-up and release of the Li+ material
by the spinel lattice, i.e., from an almost completely reversible
intercalation.
FIG. 3 shows a typical charge/discharge potential curve for a
secondary cell according to the present invention, with a positive
LiMn.sub.2 O.sub.4 electrode and a negative Li+ intercalating
carbon electrode. As shown, by far the greatest part of the
capacity, as in the above described cases with a current (i) of 0.1
CA (for 10 hours), is discharged at a cell potential which declines
from 4 volts to 3 volts.
The electrolyte was in each case a 1-normal solution of LiAsF.sub.6
in propylene carbonate.
It will therefore be understood that various changes in the
details, materials and arrangement of parts which have been herein
described and illustrated in order to explain the nature of this
invention may be made by those skilled in the art within the
principle and scope of the invention as expressed in the following
claims.
* * * * *